Facilitating Proactive Stator Winding Maintenance Using Partial Discharge Patterns

Failures in stator windings usually occur due to four factors (the so-called TEAM factor in IEC standard 60505); all of them have the potential to generate partial discharges (PD) and influence the life expectancy of the winding. Assessment of the winding condition using continuous online PD measurements allows early detection of weak spots in the insulation, identify the need for maintenance repairs and therefore reduce the risk of unexpected failures. For example, repairing the junction between the conductive and voltage grading coating is often possible when the underlying mica insulation is not affected. Typical interconnected high voltage assets in a power generation plant are the combination of generator - isolated phase bus - generator step-up transformer. PD activity from interconnected HV assets overlap and can result in false alarms and erroneous conclusions on the origin of the fault sources. This paper starts with a brief description of the permanent online monitoring system used in this study, which uses conventional and unconventional PD sensors configured in a specific manner to capture separately PD activity occurring in different sections of an interconnected HV asset, and use them in PD source characterization and insulation diagnosis. PD patterns measured for normally deteriorated generators are analyzed, and the correlation between the causes of the defects and the PD patterns are shown. Using case examples drawn from several years of experience with continuous online PD measurements, it is shown that it aids not only early detection of faults but also an effective way to reduce required plant maintenance activities, cut operating costs, and prevent unplanned shutdowns.